The present invention relates to a method of manufacturing a die designed for the duplication of a video frequency signal recording. It relates in particular to the reproduction of flexible or rigid discs comprising hollow or projecting impressions capable of being read out optically. These impressions, for example, form a succession of diffractive elements which translate television signals by virtue of their mutual spacing and non-uniform lengths. The impression takes the form of an extremely fine engraved formation (hollows or projections of the order of one micron, interval between turns of the track, of the same order).
The technique of information broadcasting, through the medium of a physical data carrier which enables storage and later reconstitution to be carried out in the case hereinbefore referred to, comprises the following three stages: (1) the engraving of a "master impression" on a first data carrier, the latter being for example a rigid disc receiving the impressions in the form of a spiral track; (2) manufacture of a die which is a "counterpart" of the recording; (3) duplication of the master impression by the repeated pressing of as many flexible or rigid discs as it is required to produce.
In a first known method, the first stage consists in engraving a photosensitive resin film, commenced in exposing it by a fine laser beam modulated by the signal which is to be recorded and focussed on to the resin, this being followed by development of the resin. The second stage is carried out in two steps using electrolysis, the first step involving deposition of silver by electrolytic techniques upon an insulating substrate (the "ELECTROLESS" process), the second involving the deposition of nickel upon the silver layer. The third stage is a conventional pressing operation producing hollow impressions in a transparent thermoplastic material. This method does not give a very few accurate engraved formation in the photosensitive resin film.
In a second known method, the accuracy is better because a laser is used to engrave a very thin volatilisable film formed upon a film of photosensitive resin which is subsequently exposed and subjected to a complementary chemical operation in order to achieve the optimum depth. The second and third stages are similar to those of the first method.
In a third known method, engraving is carried out in the manner employed in the second method referred to earlier but a film is used which is constituted by a volatisable organic polymer covered with a thin gold film. Because of the presence of this electrically conductive metal, direct electrolytic deposition can be carried out without using any photosensitive resin and this eradicates one stage although it yields a die which is a replica of that obtained in the previous methods so that it is necessary to produce a counterpart by electrolysis, if it is required to produce impressions of hollow kind after the pressing operation. In the three methods described hereinbefore, the die has to have a solid and rigid base which can be achieved by continuing electrolytic deposition of metal until a substantial thickness has been built up. This is a time consuming and expensive technology. The invention makes it possible to overcome these drawbacks by avoiding the need for any metal deposition by electrolytic methods.